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1.
Collection of samples of suspended sediment transported by streams and rivers is difficult and expensive. Emerging technologies, such as acoustic backscatter, have promise to decrease costs and allow more thorough sampling of transported sediment in streams and rivers. Acoustic backscatter information may be used to calculate the concentration of suspended sand-sized sediment given the vertical distribution of sediment size. Therefore, procedures to accurately compute suspended sediment size distributions from easily obtained river data are badly needed. In this study, techniques to predict the size of suspended sand are examined and their application to measuring concentrations using acoustic backscatter data are explored. Three methods to predict the size of sediment in suspension using bed sediment, flow criteria, and a modified form of the Rouse equation yielded mean suspended sediment sizes that differed from means of measured data by 7 to 50 percent. When one sample near the bed was used as a reference, mean error was reduced to about 5 percent. These errors in size determination translate into errors of 7 to 156 percent in the prediction of sediment concentration using backscatter data from 1 MHz single frequency acoustics.  相似文献   

2.
This paper illustrates how the acoustic Doppler current profiler （ADCP） and single-beam echo-sounder （SBES） recordings can be used for the calibration of existing software to assist in generalizing the morphodynamic processes in large rivers at key sites such as bifi.trcations and confluences. Calibration of the MIKE21C numerical model by the Danish Hydraulic Institute at the 25-km-long reach of Lower Paran~ near Rosario （Argentina） is presented. This reach includes two downstream confluences and two bifurcations. The model simulates a 2-D depth-averaged flow velocity and the related sediment fluxes to predict the bifurcation morphodynamics that affects the Paranh waterway. To investigate the river channel bathymetry, roughness, flow discharge allocation at bifurcations, suspended sediment concentration and grain size distributions, several instruments were used. These instruments included two ADCPs by Teledyne RDI working at frequencies of 600 and 1,200 kHz, a Sontek ADCP working at a frequency of 1,000 kHz and a SBES. The method to assess suspended sediment concentration and grain size distributions has been previously described. This paper focuses primarily on investigating dune morphology （by means of SBES depth measurements） and friction velocity （by means of ADCP profiling） to determine the river channel bed-roughness. The 2-D model results agree with observed values of bed-roughness, flow velocity and suspended sediment concentration distributions at the investigated sections, known data of water slope and total load of bed sediment are in good agreement with model results.  相似文献   

3.
A combination of two indirect methods to measure sediment flux is presented in this study to evaluate suspended sediment transport in a hydropower reservoir. The acoustic backscatter signal (ABS) from an Acoustic Doppler Current Profiler (ADCP) is therefore applied in pre-defined transects within the reservoir in combination with a Laser In-Situ Scattering Transmissometry – stream lined device (LISST-SL). The stationary LISST-SL derived suspended sediment concentration (SSC) measurements are used to calibrate the ABS. From the LISST-SL measurements a time series of SSC is obtained. This enables, in addition, a comprehensive data analysis to evaluate the influence of natural fluctuations of the SSC on the calculated sediment flux, which should be taken into account when assessing sediment transport. Furthermore SSC measurements are done with the LISST-SL close to the reservoir bed. In areas close to the bed no information regarding the ABS is available from the ADCP measurements due to the side-lobe interference. In various studies the information from the last three valid cells is used for extrapolation. However, as result of a comparison of the LISST-SL measurements with extrapolated SSC values from the ADCP measurements it can be seen that, especially in deep reservoirs, this method has to be adapted to the in-situ conditions.  相似文献   

4.
Large rivers have been previously shown to be vertically heterogeneous in terms of suspended particulate matter (SPM) concentration, as a result of sorting of suspended solids. Therefore, the spatial distribution of suspended sediments within the river section has to be known to assess the riverine sedimentary flux. Numerous studies have focused on the vertical distribution of SPM in a river channel from a theoretical or experimental perspective, but only a few were conducted so far on very large rivers. Moreover, a technique for the prediction of depth‐integrated suspended sediment fluxes in very large rivers based on sediment transport dynamics has not yet been proposed. We sampled river water along depth following several vertical profiles, at four locations on the Amazon River and its main tributaries and at two distinct water stages. Depending on the vertical profile, a one‐ to fivefold increase in SPM concentration is observed from river channel surface to bottom, which has a significant impact on the ‘depth‐averaged’ SPM concentration. For each cross section, a so‐called Rouse profile quantitatively accounts for the trend of SPM concentration increase with depth, and a representative Rouse number can be measured for each cross section. However, the prediction of this Rouse number would require the knowledge of the settling velocity of particles, which is dependent on the state of aggregation affecting particles within the river. We demonstrate that in the Amazon River, particle aggregation significantly influences the Rouse number and renders its determination impossible from grain‐size distribution data obtained in the lab. However, in each cross section, the Rouse profile obtained from the fit of the data can serve as a basis to model, at first order, the SPM concentration at any position in the river cross section. This approach, combined with acoustic Doppler current profiler (ADCP) water velocity transects, allows us to accurately estimate the depth‐integrated instantaneous sediment flux. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

5.
Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean.Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler(ADCP) to monitor bed shear stress,applying a prescribed boundary layer model,previously used for discharge estimation.The model parameters include the local roughness length and a dip correction factor to account for sidewall effects.Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data.We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP.To validate the results,co-located coupled ADCPs were used to infer bed shear stress,both from Reynolds stress profiles and from mean velocity profiles. From HADCP data collected over a period of 1.5 years,a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River,East Kalimantan,Indonesia.A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs.The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow,which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP.Bed shear stresses from the coupled ADCP are representative of a much more localized flow,while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales.Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods,and highlight the robustness of the method to uncertainty in the estimates of the roughness length.  相似文献   

6.
This paper presents the results of a 3-year-long (November 2004–November 2007) study based on the use of acoustic Doppler current profilers (ADCPs) to estimate the solid transport through the three inlets of Venice lagoon. In each of the three inlets instruments were mounted both on survey boats and deployed on the channel bed. The three bottom-mounted ADCPs were positioned in the central part of the inlets, continuously monitoring vertical profiles in the water column. Periodic transects along the investigated sections were collected by the boat-mounted ADCP. Both installations measured current speed and acoustic backscatter intensity. The latter expresses the attenuation of acoustic energy due to material in the water column.  相似文献   

7.
Suspended sediment is supplied from river bed sediment in Hiyamizusawa Brook, Hokkaido, Japan, during the early snowmelt season. The stirring up of fine grains from the river bed is an important control of the time variation of suspended-sediment flux. In this stream, about 10 per cent of the river bed is covered with sand sediment, 80 per cent with cobbles and/or pebbles and the remaining 10 per cent is exposed bedrock. A model previously used to explain the stirring up of fine grains within a cobble and pebble bed is applied to a sand bed, with the modification that fine grains in a sand bed are assumed to be stirred up from the tractive layer formed on the surface, whereas those in a cobble and pebble bed are assumed to be stirred up from the gaps formed by the selective movement of pebbles on the river bed. The lift force acting at the river bed is estimated from the bed shear stress, and the maximum grain size capable of being stirred up was calculated from the lift force. Consequently, the amount of fine material stirred up from the river bed is estimated from the grain size distribution of river bed sediment, and the suspended-sediment flux is thus calculated. All stirred-up fines are assumed to become suspended sediment. The simulated time variation of suspended-sediment concentration was similar to that obtained in the field study. The calculated grain size of suspended sediment was also equivalent to the field data.  相似文献   

8.
Field data from the Rio Paraná, Argentina, are used to examine patterns of suspended sediment transport over a sand dune. Measurements of three‐dimensional velocity are made with an acoustic Doppler current profiler whilst suspended sediment concentration and particle size have been quantified using a laser in situ sediment scattering transmissometer. Suspended sediment concentration and streamwise and vertical sediment flux are highest close to the bed, with an upward vertical flux over the stoss side of the dune and downward flux over the lee side. Suspended sediment concentrations are higher over the crest compared with the trough and suspended sediment is coarsest near the bed. About 17% of the suspended‐load transported over the crest is deposited in the lee side before it reaches the trough. Most of this deposited sand is coarser sediment that originates close to the bed over the crest, a result consistent with simulations based on the model of Mohrig and Smith (Water Resources Research 1996; 32: 3207–3217) for the excursion lengths of sediment dispersed in the lee side of a dune. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

9.
The relationship between turbulent fluid motions and sediment particle motions over mobile sand dunes was investigated by using a laser Doppler velocimeter and an acoustic backscatter system in laboratory experiments performed at the USDA-ARS-National Sedimentation Laboratory. Profiles of acoustic backscatter from particles and at-a-point turbulence data were collected while translating both measurement devices downstream at the speed of mobile dune bedforms. The resulting data set was used to examine the frequency （recurrence frequency） at which the fluctuating backscatter and fluid velocity signals exceeded magnitude thresholds based on the standard deviation （σ） of the local velocity and the magnitude the acoustic signal resulting from backscatter from suspended particles. The slope of the downstream and vertical velocity recurrence frequencies generally indicated a gradually increasing recurrence time with increasing elevation. The recurrence frequency for acoustic backscatter data was not strongly variable with elevation. The closest correspondence between the recurrence frequencies of sediment backscatter and vertical velocities at the 1σ magnitude threshold was in a region defined by X/L〈0.4 and y〈6 cm. The downstream velocity was most closely related to backscatter in a small region at 0.4〈X/L〈0.8 and less than 3-4 cm from the bed.  相似文献   

10.
Most rivers in Taiwan are intermittent rivers with relatively steep slopes and carry rapid sediment‐laden flows during typhoon or monsoon seasons. A series of field experiments was conducted to collect suspended load data at the Tzu‐Chiang Bridge hydrological station of the lower Cho‐Shui River, which is a major river with the highest sediment yield in Taiwan. The river reach was aggrading with a high aspect ratio during the 1980s. Because of sand mining and extreme floods, it was incised and has had a relatively narrow main channel in recent years. The experimental results indicated that typical sediment transport equations can correctly predict the bed material load for low or medium sediment transport rates (e.g. less than about 1000 tons/day‐m). However, these equations far underestimate the bed material load for high sediment transport rates. The effects of cross‐sectional geometry change (i.e. river incision) and earthquakes on the sediment load were investigated in this study. An empirical sediment transport equation with consideration of the aspect ratio was also derived using the field data collected before and after river incision. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

11.
The particle size distribution of bed materials in the sandy river bed of alluvial rivers is important in the study of topics such as friction, river bed evolution, erosion, and siltation. It also can reflect the dependency relation between river bed sediment and flow intensity. In this paper, the critical pattern of sediment movement in the near-wall region of a sandy river bed was analyzed. According to the principle of momentum balance, the critical settling-rising condition of bed material in a sandy river bed was found to be instantaneous turbulent velocity equal to 2.7 times the sediment settling velocity in quiescent water. Based on a vertical instantaneous turbulent velocity with a Gaussian distribution, a theoretical relation for calculating the particle size distribution of bed materials in a sandy river bed without pre-known characteristic grain sizes was developed by solving a stochastic equation. The for-mula is verified using measured data, and the results show that the proposed formula was in accordance with the measured data. This study has theoretical significance and practical value for determining the bed material particle size distribution of the sandy bed of alluvial rivers.  相似文献   

12.
Many urban rivers receive significant inputs of metal‐contaminated sediments from their catchments. Restoration of urban rivers often creates increased slack water areas and in‐channel vegetation growth where these metal‐contaminated sediments may accumulate. Quantifying the accumulation and retention of these sediments by in‐channel vegetation in urban rivers is of importance in terms of the planning and management of urban river restoration schemes and compliance with the Water Framework Directive. This paper investigates sediment properties at four sites across three rivers within Greater London to assess the degree to which contaminated sediments are being retained. Within paired restored and unrestored reaches at each site, four different bed sediment patch types (exposed unvegetated gravel, sand, and silt/clay (termed ‘fine’) sediments, and in‐channel vegetated sediments) were sampled and analysed for a range of metals and sediment characteristics. Many samples were found to exceed Environment Agency guidelines for copper (Cu), lead (Pb) and zinc (Zn) and Dutch Intervention Values for Cu and Zn. At all sites, sediments accumulating around in‐channel vegetation were similar in calibre and composition to exposed unvegetated fine sediments. Both bed sediment types contained high concentrations of pseudo‐total and acetic acid extractable metal concentrations, potentially due to elevated organic matter and silt/clay content, as these are important sorbtion phases for metals. This implies that the changed sediment supply and hydraulic conditions associated with river restoration may lead to enhanced retention of contaminated fine sediments, particularly around emergent plants, frequently leading to the development of submerged and emergent landforms and potential river channel adjustments. High pseudo‐total metal concentrations were also found in gravel bed sediments, probably associated with iron (Fe) and manganese (Mn) oxyhydroxides and discrete anthropogenic metal‐rich particles. These results highlight the importance of understanding the potential effects of urban river restoration upon sediment availability and channel hydraulics and consequent impacts upon sediment contaminant dynamics and storage. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

13.
Accurately measuring sediment flux in large rivers remains a challenge due to the spatial and temporal cross‐sectional variability of suspended sediment concentrations in conjunction with sampling procedures that fail to accurately quantify these differences. This study presents a field campaign methodology that can be used to improve the measurement of suspended sediment concentrations in the Amazon River or similarly large rivers. The turbidity signal and Rouse model are together used in this study to define the spatial distribution of suspended sediment concentrations in a river cross‐section, taking into account the different size fractions of the sediment. With this methodology, suspended sediment fluxes corresponding to each sediment class are defined with less uncertainty than with manual samples. This paper presents an application of this methodology during a field campaign at different gauging stations along a 3,000‐km stretch of the Solimões/Amazon River during low water and flood periods. Vertical concentration profiles and Rouse model applications for distinctive sediment sizes are explored to determine concentration gradients throughout a cross‐section of the river. The results show that coupling both turbidity technology and the Rouse model may improve our understanding of the spatial distribution of different sediments fractions sizes in the Solimões/Amazon River. These data are very useful in defining a pertinent monitoring strategy for suspended sediment concentrations in the challenging context of large rivers.  相似文献   

14.
We present detailed observations of internally generated turbulence in a sheared, stratified natural flow, as well as an analysis of the external factors leading to its generation and temporal variability. Multi-month time series of vertical profiles of velocity, acoustic backscatter (0.5 Hz), and turbulence parameters were collected with two moored acoustic Doppler current profilers (ADCPs) in the Hudson River estuary, and estuary-long transects of water density were collected 30 times. ADCP backscatter is used for visualization of coherent turbulent structures and evaluation of surface wave biases to the turbulence measurements. Benefits of the continuous long-term turbulence record include our capturing: (1) the seasonality of turbulence due to changing riverflow, (2) hysteresis in stratification and turbulence over the fortnightly cycle of tidal range, and (3) intermittent events such as breaking internal waves. Internal mixing layers (IMLs) are defined as turbulent regions above the logarithmic velocity layer, and the bottom boundary layer (BBL) is defined as the continuously turbulent range of heights above the bed. A cross-correlation analysis reveals how IML and BBL turbulence vary with stratification and external forcing from tidal range, river flow, and winds. Turbulence in both layers is maximal at spring tide and minimal when most stratified, with one exception—IML turbulence at a site with changing channel depth and width is maximal at times of maximum stratification and freshwater input.  相似文献   

15.
A large number of rivers are frozen annually, and the river ice cover has an influence on the geomorphological processes. These processes in cohesive sediment rivers are not fully understood. Therefore, this paper demonstrates the impact of river ice cover on sediment transport, i.e. turbidity, suspended sediment loads and erosion potential, compared with a river with ice‐free flow conditions. The present sediment transportation conditions during the annual cycle are analysed, and the implications of climate change on wintertime geomorphological processes are estimated. A one‐dimensional hydrodynamic model has been applied to the Kokemäenjoki River in Southwest Finland. The shear stress forces directed to the river bed are simulated with present and projected hydroclimatic conditions. The results of shear stress simulations indicate that a thermally formed smooth ice cover diminishes river bed erosion, compared with an ice‐free river with similar discharges. Based on long‐term field data, the river ice cover reduces turbidity statistically significantly. Furthermore, suspended sediment concentrations measured in ice‐free and ice‐covered river water reveal a diminishing effect of ice cover on riverine sediment load. The hydrodynamic simulations suggest that the influence of rippled ice cover on shear stress is varying. Climate change is projected to increase the winter discharges by 27–77% on average by 2070–2099. Thus, the increasing winter discharges and possible diminishing ice cover periods both increase the erosion potential of the river bed. Hence, the wintertime sediment load of the river is expected to become larger in the future. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

16.
《国际泥沙研究》2021,36(6):723-735
This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E0), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.  相似文献   

17.
This methods paper details the first attempt at monitoring bank erosion, flow and suspended sediment at a site during flooding on the Mekong River induced by the passage of tropical cyclones. We deployed integrated mobile laser scanning (MLS) and multibeam echo sounding (MBES), alongside acoustic Doppler current profiling (aDcp), to directly measure changes in river bank and bed at high (~0.05 m) spatial resolution, in conjunction with measurements of flow and suspended sediment dynamics. We outline the methodological steps used to collect and process this complex point cloud data, and detail the procedures used to process and calibrate the aDcp flow and sediment flux data. A comparison with conventional remote sensing methods of estimating bank erosion, using aerial images and Landsat imagery, reveals that traditional techniques are error prone at the high temporal resolutions required to quantify the patterns and volumes of bank erosion induced by the passage of individual flood events. Our analysis reveals the importance of cyclone‐driven flood events in causing high rates of erosion and suspended sediment transport, with a c. twofold increase in bank erosion volumes and a fourfold increase in suspended sediment volumes in the cyclone‐affected wet season. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

18.
Flow, sediment transport and bed deformation in alluvial rivers normally exhibit multiple time scales. Enhanced knowledge of the time scales can facilitate better approaches to the understanding of the fluvial processes. Yet prior studies of the time scales are based upon the concept of sediment transport capacity at low concentrations, which however is not generally applicable. This paper presents new formulations of the time scales of fluvial flow, suspended sediment transport and bed deformation, under the framework of shallow water hydrodynamics, non-capacity sediment transport and the theory of characteristics for the hyperbolic governing equations. The time scale of bed deformation in relation to that of flow depth is demonstrated to delimit the applicability region of mathematical river models, and the time scale of suspended sediment transport relative to that of the pertinent flow information is analyzed to address if the concept of sediment transport capacity is applicable. For shallow flows with high sediment concentrations, bed deformation may considerably affect the flow and a fully coupled model is normally required. In contrast, for deep flows at low sediment concentrations, a decoupled model is mostly justified. This pilot study of the time scales delivers a new theoretical basis, on which the interaction between flow, suspended sediment transport and bed deformation can be potentially better characterized.  相似文献   

19.
Dam construction in the 1960s to 1980s significantly modified sediment supply from the Kenyan uplands to the lower Tana River. To assess the effect on suspended sediment fluxes of the Tana River, we monitored the sediment load at high temporal resolution for 1 year and complemented our data with historical information. The relationship between sediment concentration and water discharge was complex: at the onset of the wet season, discharge peaks resulted in high sediment concentrations and counterclockwise hysteresis, while towards the end of the wet season, a sediment exhaustion effect led to low concentrations despite the high discharge. The total sediment flux at Garissa (c. 250 km downstream of the lowermost dam) between June 2012 and June 2013 was 8.8 Mt yr‐1. Comparison of current with historical fluxes indicated that dam construction had not greatly affected the annual sediment flux. We suggest that autogenic processes, namely river bed dynamics and bank erosion, mobilized large quantities of sediments stored in the alluvial plain downstream of the dams. Observations supporting the importance of autogenic processes included the absence of measurable activities of the fall‐out radionuclides 7Be and 137Cs in the suspended sediment, the rapid lateral migration of the river course, and the seasonal changes in river cross‐section. Given the large stock of sediment in the alluvial valley of the Tana River, it may take centuries before the effect of damming shows up as a quantitative reduction in the sediment flux at Garissa. Many models relate the sediment load of rivers to catchment characteristics, thereby implicitly assuming that alterations in the catchment induce changes in the sediment load. Our research confirms that the response of an alluvial river to external disturbances such as land use or climate change is often indirect or non‐existent as autogenic processes overwhelm the changes in the input signal. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

20.
We deployed bottom-mounted quadrapod equipped with acoustic Doppler current profiler (ADCP), acoustic Doppler velocimeter (ADV), and optical backscatter sensor (OBS) over two semidiurnal tidal cycles along the western coast of the Yellow Sea, China. In combination with shipboard profiling of CTD and LISST-100, we resolved the temporal and spatial distributions of tidal currents, turbulent kinetic energy (TKE), suspended sediment concentration (SSC) and particle size distributions. During the observations, tidal-induced bottom shear stress was the main stirring factor. However, weak tidal flow during the ebb phase was accompanied by two large SSC and median size events. The interactions of seiche-induced oscillations with weak ebb flow induced multiple flow reversals and provided a source of turbulence production, which stripped up the benthic fluff layers (only several millimeters) around the Jiaozhou Bay mouth. Several different methods for inferring mean suspended sediment settling velocity agreed well under peak currents, including estimates using LISST-based Stokes’ settling law, and ADCP-based Rouse profiles, ADV-based inertial-dissipation balance and Reynolds flux. Suspended particles in the study site can be roughly classified into two types according to settling behavior: a smaller, denser class consistent with silt and clay and a larger, less dense class consistent with loosely aggregated flocs. In the present work, we prove that acoustic approaches are robust in simultaneously and non-intrusively estimating hydrodynamics, SSC and settling velocities, which is especially applicable for studying sediment dynamics in tidal environments with moderate concentration levels.  相似文献   

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